What makes 'cos' unique?

Note: the category and title are both clues and you may need to use some twisted logic as the answer is not straightforward

Umm... it's the only 3-letter sequence using c, o, and s, in that exact order. How's THAT for twisted logic?
Edited on September 19, 2003, 5:40 pm

Posted by Happy on 2003-09-19 17:38:25